Expansions of several triplet nucleotide repeats underlie the genetic basis for a diverse group of neurological disorders. While the pathologies of these disorders vary greatly, they share a common etiology;the instability and expansion of triplet nucleotide repeats from small repetitions, usually under 30 repeats, to far greater repetitions, anywhere from 40 to hundreds of repeats. The goal of this project is to identify novel genetic determinants that modulate the genetic instability of triplet CAG repeats. To accomplish this, an in vivo screen for triplet repeat instability will be developed using Drosophila as the model system. Using Drosophila to screen for modifiers of instability is a particularly attractive approach in that most of the Drosophila genome has been targeted either by classic mutagenesis or by in vivo siRNAs. In the first aim, a Drosophila model for Spinocerebellar ataxia 1 (SCA1), containing the full-length human ataxini gene with 82 CAG repeats, will be modified to induce a baseline instability in the CAG repeat tract. Induction of instability will be optimized by manipulation of transcription levels and genomic location of the transgene. Instability will be measured by single fly PCR and the lines with the greatest instability will be used as an assay to screen for modifiers of CAG repeat instability. The second aim will explore the possibility of developing a rapid, clinically relevant readout for the CAG repeat instability assay. Our lab has found that long CAG repeat tracts can dramatically affect the morphology of photoreceptors when expressed in the eye, and greatly impair flight when expressed in motor neurons. Molecular data from single fly PCR will be correlated to morphological or behavioral phenotypes to ascertain whether alterations to repeat length quantitatively affect these phenotypes;allowing for their use to sensitively monitor changes to the CAG repeat tract in the ataxini gene. For the final aim, a screen will be carried out with libraries of mutant flies that cover the entire genome, to identify novel contributors to CAG repeat instability. Public Health Relevance: This project aims to target a diverse group of devesting neurological disorders by tackling the common source of pathogenesis, the instability and expansion of CAG nucleotide repeats. Modifiers that enhance instability will reveal novel pathways that contribute to disease, and those that suppress instability, or specifically contract the repeats might be candidate drug targets that could exploited to prevent or delay the onset of neurological disorders caused by expanded CAG nucleotide repeats.